Scanning electron microscopes comprise a source of primary electrons, a condenser lens regulating a primary electron beam, an aperture lens, an objective lens, scanning and centering elements and a detector. The condenser lens regulates the primary electron beam. The objective lens is usually electromagnetic. Such lens consists of a coil with a current passing through the coil and of a yoke made of magnetic material forming a part of the magnetic circuit of the lens. The coil can have one, two or more windings, which can be used for maintaining a constant heat output of the coil while changing the intensity of the magnetic field generated by the lens. The magnetic field which forms the electron beam is generated between the so-called polepieces in the place of the interruption of the yoke. The electromagnetic lens can be a conventional lens or an immersion lens. The conventional lens has two polepieces with an axial gap in which the magnetic field of the lens locally affects the primary electron current (electron beam) and the magnetic field does not extend significantly to the area where the sample is located. The immersion lens can either have single polepiece, in such case the magnetic field closes over the microscope chamber, or the lens can have two polepieces with a radial gap. When using the immersion lens, the examined sample is immersed into the magnetic field generated by the lens. The advantage of the immersion lenses over the conventional ones lies in decrease in the optical aberrations of imaging and in improvement of the resolution of the electron microscope.
The electromagnetic lenses can be also substituted by electrostatic lenses which utilize electrodes instead of coils, optionally a combined lens can be created by combining coils, magnetic circuit and electrodes.
A drawback of a common scanning electron microscope is that it can usually utilize only one objective lens with given properties and therefore has a very limited number of levels of configuration freedom of the microscope imaging modes. If, for example, the objective lens is optimized for high resolution, it has usually relatively small field of view and a small depth of focus. The aperture diaphragm and condenser adjust the aperture angle of the primary beam together with the desired current of electrons passing through the aperture diaphragm. However, for the objective lens focusing the primary electron beam onto the sample, there is only a relatively small range of the electron current which corresponds to the aperture angle required for the optimal resolution in this arrangement. Therefore, when changing the current of passed electrons (and thereby also when changing the aperture angle), the resolution of the objective lens decreases.
If, for example, the objective is set for the optimal aperture angle corresponding to the small current, it loses its resolution for high currents.
This drawback is eliminated in U.S. Pat. No. 5,124,556. The document generally describes the arrangement of two condenser lenses, aperture diaphragm and two objective lenses, out of which one objective lens is auxiliary and serves for the aperture angle control of a charged particle beam incident on the examined sample. Said arrangement allows maintaining optimal aperture with a wider range of currents of the primary electron beam impinging the sample. The drawback of this arrangement is a relatively small field of view.
U.S. Pat. No. 7,705,298 describes a device comprising three condenser lenses (two electromagnetic and one electrostatic) and an objective lens which is a combination of an electromagnetic immersion double polepiece lens and an electrostatic lens. These objective lenses shape the electromagnetic and electrostatic field for focusing the primary electron beam onto the sample. This device further comprises quickly responding electromagnetic auxiliary lens which serves for quick change in the focus of the beam on the vertically topographic sample. However, this document does not disclose the possibility to use multiple imaging modes, and the auxiliary lens cannot serve, due to its properties, for imaging, but only as a supplement to the main objective lens.
Patent EP0708975 describes an arrangement of two objective lenses, the objective lens located closer to the sample being the immersion lens and the objective lens further from the sample being the conventional lens. However, these two lenses do not work in cooperation, they switch between imaging by means of the immersion lens for the highest resolution and imaging by means of the conventional lens for the mode without magnetic field on the sample, which is suitable for example for samples from magnetic materials. Moreover, the possibility of using multiple imaging modes is not mentioned.
The device disclosed in the patent JP2969219 comprises two condenser lenses and two objective lenses. The objective lenses are electromagnetic and they are arranged identically to the ones in the solution disclosed in EP0708975. One electromagnetic objective lens is conventional, the other electromagnetic objective lens is the immersion lens. The drawback of both of these solutions is that both lenses are arranged as close to the sample as possible to achieve the biggest resolution and therefore it is not possible to achieve large field of view which is important mainly for easy navigation on the sample.
U.S. Pat. No. 7,223,983 describes an arrangement of at least one condenser lens, two deflection coils for regulation of the electron beam direction, subsequently, the detector of secondary electrons, scanning coils, auxiliary objective lens and objective lens are arranged. Said elements of the microscope configuration have electromagnetic character. The device allows beam deflection at large angle with the image undistorted. The possibility of using multiple imaging modes is not mentioned here either.
Therefore, the possibility of achieving imaging modes for high resolution, increased depth of focus and large field of view in the same device and for range of electron currents is not provided by any of the known devices for ultra-high resolution.